The invention relates to a harmonic drive and internal geared wheels for such drives according to the respective preamble of the main claims, as known in this respect from DE 1 96 50 716 C1 relating to an eccentric drive with overload protection.
The function of such an eccentric or harmonic drivexe2x80x94also known as a circular spline drivexe2x80x94as a very greatly reducing, self-locking system with an output shaft coaxial with the drive shaft is based on the fact that a rotating wave generator radially deforms an internal geared wheel rim, also referred to as a flexspline, orbitally and in this way orbitally presses its outer lateral surface outward locally against the hollow-cylindrical inner lateral surface of slightly greater circumference of a stationary, dimensionally stable support ring. Consequently, the internal geared wheel itself or its wheel rim mounted rotatably thereon rolls in the support ring non-positively via friction surfaces or positively via toothings, the wheel or its rim rotating more slowly than the motor-driven driving core of the wave generator according to the difference in circumference. This rotary motion, which is greatly slowed in relation to the drive, is preferably transmitted via the outer toothing of the wheel rim to the inner toothing of a further hollow-cylindrical outer ring of the output ring which is concentric with the support ring but not stationary. The drive of the wave generator usually takes place via a coaxially flanged-on high-speed, and therefore inexpensively procurable, extra-low-voltage d.c. motor, the rotation of which is thus reduced to a very much slower rotary motion of correspondingly greater torque.
In the embodiments of such harmonic drives known from German Utility Model 2 96 14 738 and described in greater detail in the article xe2x80x9cGenial einfach [Ingeniously simple]xe2x80x9d by H. Hirn (KEM Antriebstechnik, issue November 1996), a non-round (in axial cross section roughly triangular or preferably oval) driving core, as the wave generator, is rotated concentrically in the hub of the radially deformable internal geared wheel. Dimensionally stable spokes, acting as radially oriented push rods, between the hub deformed radially by the driving core and the likewise radially deformable, externally toothed rim of this internal geared wheel cause the outer toothing, according to its orbital radial deformation, to enter into engagement with the inner toothing of the support ring at any one time over only a limited curved segment rolling therein.
The dimensional stability of the spokes must be guaranteed above all in the radial direction in order for it to be possible to transmit the orbital radial deformation of the hub (referred to as an internal geared wheel in the prior publication constituting the generic type) generated by the wave generator to the rim (referred to as the external geared wheel there). However, during the rolling operation of the rim in the support ring, which takes place under load, the spokes of this segment, which are linear there and described as flexible in the peripheral direction, are also subjected to bending stress at both their ends. If they then bend in the peripheral direction out of the radial direction, the diameter of the rim decreases until it comes out of engagement with the support ring. The spokes thus act as overload protection on the harmonic drive.
The invention is based on the technical problem of, while retaining its advantages in terms of apparatus and application, to develop the previously known harmonic drive or its internal geared wheel to the effect that it can be designed considerably more flexibly in particular with regard to the kinetic conditions when the overload protection responds and will thus have a greater variety of uses.
According to the invention, this object is achieved by the harmonic drive according to the generic type and its internal geared wheel being designed with the features of the respective main claim. Accordingly, the internal geared wheel is still equipped with spokes, which are no longer dimensionally stable in the peripheral direction at critical stress, a torque-dependentxe2x80x94and optionally a rotation-direction-dependentxe2x80x94response of this safeguarding function against overloading then being specifically constructionally influencable with regard to many parameters owing to the special shaping or material combination, according to the invention, of the spokes. As mentioned, such restorable deformation occurs depending on the flexural rigidity of the spoke when that segment of the wheel rim assigned to its radial orientation is directly in engagement with the support ring and a constructionally specified critical bending stress of this spoke is on that account exceeded during the rolling operation.
In this connection, the deformability of the spokes can therefore be designed so as to be independent of the direction of rotation of the driving core in the wheel hub, so that, as it were, the function of an overload protection or slipping clutch acting in both drive directions is brought about. On the other hand, this function can also be defined so as to be dependent on the direction of rotation by the spoke having, owing to its geometry or material combination, a different flexural rigidity in one direction of rotation than in the opposite onexe2x80x94up to the free-wheeling characteristic in one direction with power transmission in only the specified other direction, the working direction. This is because the deformation of the spoke shaft, which is subject to overload or which is now no longer flexurally rigid in the counter-rotation direction, shortens the radially effective spoke length of the internal geared wheel and thus the local radial deformation of the wheel rim, that is to say its toothing engagement with the support wheel, which then ratchets through and thus comes out of engagement, as a result of which the power and motion transmission via the harmonic drive is interrupted.
Consequently, by means of the measures according to the invention, that is to say as a result of influencing the toothing engagement, the toothing parameter on the wheel rim (flexspline) is modified in such a way that, when the specified maximum torque is exceeded, ratcheting-over of the toothing occurs. This leads to the drive coming to a standstill in the event of overload, in which connection this torque-limiting by means of buckling spokes up to free-wheeling functioning owing to spokes being inclined in relation to the radials or owing to spokes folding down can also be direction-dependent, without additional functional assemblies having to be introduced for this before or after the harmonic drive.